Method of salt bath nitriding for producing iron member having improved corrosion resistance and iron parts

ABSTRACT

A new nitriding process by using a salt bath to produce iron and steel parts having excellent abrasion resistance and corrosion resistance are explained. A iron lithium complex oxide layer are formed at the outermost surface of the iron part by immersing the iron and steel parts in a salt bath containing cationic component of Li, Na and K and anionic components of CNO − and CO 3   2− , where hydroxide compound selected from lithium hydroxide, sodium hydroxide and potassium hydroxide are added to the salt bath. Materials being in a hydrated state or in a free water containing state can be used for preparation or replenishing of the salt bath. An moistend air of (1×10 −2  kg·H 2 O)/(1 kg dry air) can be used for mixing the salt bath. Containing ratio of Li, Na, K is preferable where a solidifying temperature of the mixture of carbonates of Li, Na, K in that ratio is lower than 500° C. It is preferable that the mol ratio of Na and K is to be 2:8˜8:2, the content of CNO −  is to be 5˜35 wt %, the content of CN −  in the salt bath is less than 2 wt % and the temperature of the salt bath is to be 450˜650° C.

FIELD OF THE INVENTION

[0001] This invention relates to an improvement of corrosion resistanceof iron and steel parts obtained by nitriding in a salt bath, which alsoprovides high abrasion resistance and high strength against fatigue.

PRIOR ART

[0002] Nitriding process in salt bath, which forms a nitrided layer on asurface of iron and steel materials, has been utilized to improvestrength of the surface of those iron and steel materials,thereby toenhance abrasion resistance and strength against fatigue of thosematerials. The nitrided layer formed by the above-described processinghas also a function to prevent a corrosion loss of the materials.Therefore, if it is the case that corrosion resistance of usual improvedlevel is required, this process may be completed by employing aconventional nitriding process in salt bath.

[0003] However, for a use where corrosion resistance at a high level asin hard chromium plating is required, which is a competitive surfacehardening process, further processing must be made in addition to thenitriding process in salt bath.

[0004] Improvement of corrosion resistance of iron and steel parts bynitriding process has been reported in JP56-33473A, JP60-211062A,JP5-263214A, JP05-195194A, JP7-62522A, JP-7-224388A, etc.

[0005] In JP56-33473A and JP07-22438A, a combined processing ofnitriding process and oxidation bath process is proposed as a method toimprove corrosion resistance. The corrosion resistance obtained by thiscombined processing was found to be as equivalent or superior than thatobtained by hard chromium plating process in salt water spray test.

[0006] However, since corrosion resistance level obtained by saidcombined processing with oxidation bath widely varies and this methodwas usually not applied in view of a quality control(lower limit valuecontrol of products).

[0007] Further a method of using a wax following to a nitriding processand oxidation bath process, and a method to apply a polymer coating havebeen proposed in, for example, JP05-195194A and JP05-263214A.

[0008] Said two methods mentioned above are aiming at, as one aspect,lower a abrasion coefficient of the material and then to enhanceabrasion resistance of the material by way of applying either wax orpolymer coating to the material, and, as another aspect, sealing orcovering an oxide layer of the material by coating with wax or polymerthereby to enhance corrosion resistance and stability of the material.These two methods enable to improve and stabilize the materialproperties, such as abrasion resistance, strength against fatigue andcorrosion resistance.

[0009] However, it is not a easy way to accept in views of investment,production efficiency and cost to incorporate a process of coating ofwax or polymer in addition to said oxidation bath process following tothe nitriding. Based on such a background, the following was proposed.

[0010] In JP07-62522A, another nitriding method for providing corrosionresistance to iron and steel parts has been proposed. This method formsan oxide layer on the nitrided layer by performing anodic electrolysisduring nitriding process. Since this method requires a single salt bath,it is expected that great advantages in the productivity and productioncost can be attained by replacing of conventional two-step process ofnitriding process and oxidation bath.

[0011] However, the process of anodic electrolysis is executed by usingthe opposite electrode as a cathode. And, due to a cathodic reaction atthe opposite electrode, cyanate compound in the salt bath is reduced toproduce cyanide compounds, and accordingly, concentration of thepoisonous cyanide compound in the salt bath tends to be increased thanin the salt bath where no electrolysis is executed.

[0012] In addition, for carrying out an appropriate operation, currentdensity at each sites of the iron and steel parts must be controlled ina predetermined range. For this purpose, close attentions are necessaryto an arrangement of the electrode and the iron and steel parts to beprocessed. Furthermore, if the iron and steel parts to be processed hasa unsuitable configuration for electrolysis such as having deep holes orbag-shaped holes, employment of this method would be difficult.Therefore, iron and steel parts to be processed by this method must belimited.

[0013] Based on the background as described above, there has been arequirement to establish a new method of nitriding process, whichcomprises single step, does not require an electrolysis, and can provideiron and steel parts having satisfactory abrasion resistance andcorrosion resistance.

DISCLOSURE OF THE INVENTION

[0014] It is disclosed in JP58-77567A that in a nitriding process usinga salt bath comprising anionic components of CNO⁻ and CO₃ ²⁻, and twocationic components of Na⁺ and K⁺, an unexpected black-colored film insmut form having poor adhesiveness is produced on a surface of thenitrided layer when a content of a by-producted cyanide in the salt bathis low. And, it is known that this film in smut form is a magnetite(Fe₃O₄).

[0015] The inventors of the present invention carried out more differentnitriding of a steel plate using a salt bath comprising anioniccomponents of CNO⁻ and CO₃ ²⁻ and three cationic components of Li⁺, Na⁺and K⁺, where the content of the by-producted cyanide in the salt bathis kept low. In contrast to the result in JP58-77567A using a salt bathcontaining Na⁺ and K⁺ as the cationic component, inventors has obtaineda black-colored film with satisfactory adhesion to the material.

[0016] Then, the processed steel plate by the inventors was subjected toa salt water spray test to check the corrosion resistance. As a result,the steel plate by the inventors showed to have high corrosionresistance, namely more than 200 hours are required to cause the rust onthe surface of the steel plate. With this result, it is judged that theblack-colored film with satisfactory adhesion has a function to protectiron and steel parts.

[0017] With regard to the reason why this protective film is formed onthe surface of material in the salt bath containing a low concentrationof a cyanide product, the inventors are supposing as follows.

[0018] 1. Since the content of the by-producted cyanide having areducing property is low, oxidizing property of the salt bath isenhanced, thereby causing oxidation of a surface of iron to produce itsoxides in parallel to nitriding reaction by a cyanate.

[0019] 2. Since the concentration of CN⁻ having a strong power todissolve the iron is low, and since a capability of the salt bath todissolve iron oxides produced on a surface of iron is lowered, theoxides can be produced as in 1. above and an oxide film can be formed onthe outermost surface.

[0020] The inventors of this invention analyzed the film on the steelplate produced by the salt bath of three-component of Li⁺, Na⁺ and K⁺ asdescribed above by means of X-ray diffraction.

[0021] As a result, it was found that the film produced by the salt bathof three alkali metal component including lithium is an iron-lithiumcomplex oxide.

[0022] As iron-lithium complex oxides, Li₂Fe₃O₄, Li₂Fe₃O₅, Li₅Fe₅O₈,LiFe₅O₈, LiFeO₂, Li₅FeO₄, Li₂Fe_(2.4)O_(4.6) and the like have beenknown. From the analytical result by X-ray diffraction of the film,Li₂Fe₃O₄, Li₂Fe₃O₅, Li₅Fe₅O₈ and LiFe₅O₈ have been observed so far.

[0023] Reasons why the film of this iron-lithium complex oxide isadhesive and good in corrosion resistance.

[0024] In case of the salt bath of two cationic component of Na⁺ and K⁺,a film of (magnetite Fe₃O₄) in smut form with poor adhesion is producedon the steel plate. On the other hand, when the salt bath of threecationic component of Li⁺, Na⁺ and K⁺ is used, a film of iron-lithiumcomplex oxide having satisfactory adhesion property and good incorrosion resistance is formed. The inventors of the present inventionhave supposed the reason as follows.

[0025] In case of the salt bath of two component of Na⁺ and K⁺, the filmproduced onto the surface of the steel plate is magnetite(Fe₃O₄). Theboth cationic ions of Na⁺and K⁺ have a large ionic diameter. Therefore,they cannot be a constituent component of the oxide layer. Theconstituents of the magnetite are Fe²⁺, Fe³⁺ and O²⁻.

[0026] Since these ions are all multiply charged ions, it is difficultfor them to simultaneously satisfy a neutralization of electric chargesand a suitable positioning of lattice structure during formation of thefilm. And the formed film has various defects in microscopic andmacroscopic views.

[0027] In contrast thereto, the film produced onto the surface of asteel plate when using the salt bath of three component of Li⁺, Na⁺ andK⁺ is the iron-lithium complex oxide. Since Li⁺ ion has small ionicdiameter, it can be incorporated into the iron oxide film as aconstituent, thereby the iron-lithium complex oxide is produced.

[0028] Since Li⁺ is a monovalent cation, it has an important function tosimultaneously satisfy a neutralization of charges and a suitablepositioning of lattice structure during formation of a film. By virtueof this function of Li⁺, it is assumed that the film having less defectcan be formed. Incidentally, it is known that Li⁺ can move in the oxideeven at a room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a graph showing a relation between cyanate concentrationand by-producted cyanide concentration in the salt bath containing Li,Na and K

[0030]FIG. 2 is a graph showing an example of the composition of thefilm formed by the process according to the present invention.

[0031]FIG. 3 is a diagram explaining a preferable range of compositionof the salt bath.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

[0032] Based on the general experience of the inventors that a filmhaving adhesion property and corrosion resistance property can be formedonly by applying nitriding salt bath where the content of theby-producted cyanide is low in the salt bath containing of anioniccomponents of CNO⁻ and CO₃ ²⁻ and cationic components of Li⁺, Na⁺ andK⁺, a test was carried out in order to find out preferable range of thefilm-forming process.

[0033] Since it was supposed that the aimed iron-lithium complex oxidefilm will be formed when the by-producted cyanide in the salt bath is ata low concentration, in this example 1, the content of CNO⁻, which is aparent substance of producing the by-producted cyanide in the salt bath,was set at two concentration levels, that is 35 wt % as a standardconcentration and 15 wt % as low concentration. The composition of thesalt bath is shown in Table 1 below. TABLE 1 Component in Salt Bath S2-1S2-2 Li⁺ (mol %) 31 31 Na⁺ (mol %) 26.5 26.5 K⁺ (mol %) 42.5 42.5 CNO⁻(wt %) 35 15 CO₃ ⁻ Balance Balance

[0034] 60 Kg of salt mixture having composition of S2-1 in Table 1 wasplaced in a crucible made of titanium having a diameter of 350 mm and adepth of 500 mm to which a pipe for air bubbling was provided, and themixture was then melted. 35wt % of CNO⁻ was provided by coverting acarbonate according to a process shown in JP54-7502B.

[0035] The molten salt bath is maintained at 580° C. while air was blownfrom the bottom at a blowing rate of 150 L/Hr to ensure the homogeneityof the salt bath. The test was then carried out by using round bar ofcarbon steel S15C (20 mmø×8 mmt), cold rolled steel sheet SPCC (50mm×100 mm×0.8 mmt) and iron powder (surface area: 8 m²/120 g) of 60mesh. Iron powder was used for increasing in experiment the processingarea of iron materials. The carbon steel S15C and the cold rolled steelsheet SPCC were immersed in the salt bath for 90 min. at 580° C.,water-cooled, washed with tap water and dried.

[0036] The iron powder in an amount of 120g for each time was added intothe molten salt bath 5 times a day at an interval of 90 min. At the timeof the fifth addition of the iron powder, the carbon steel S15C and thecold rolled steel sheet SPCC were processed. At that time, sampling wasmade from the molten salt bath for the analysis.

[0037] At the end of the operation for one day, solid dregs in themolten salt bath were removed. The processing tests were continuouslycarried out for 8 days.

[0038] The molten salt bath of a composition as shown in S2-2 in Table 1was prepared in the same manner except the amount of CNO⁻ is adjusted to15 wt %. Then, the tests were carried out as same as in the case of themolten salt bath of S2-1. FIG. 1 shows the amount of the by-productedcyanide in the salt bath of S2-1 and S2-2 respectively.

[0039] It was determined that the content of the cyanide in both of thesalt baths S2-1 and S2-2 at the starting time was zero respectively. Inboth salt baths of S2-1 and S2-2, it was recognized that the content ofthe cyanide gradually increased with the progress of the test.

[0040] In the salt bath of S2-1, the content of the cyanide was. 0.4 wt% on the third day, and it reached to near 1.7 wt % on the eighth dayand the content is still increasing.

[0041] On the other hand, in the salt bath of S2-2, the content of thecyanide was 0.26 wt % on the third day, it reached to the peak value of0.54 wt % on the seventh day and then came to the equilibrium on theeighth day.

[0042] The external appearance of the carbon steel S15C and the coldrolled steel sheet SPCC after the test was checked. As a result, in caseof salt bath of S2-1, a black-colored surface that seems to becontaining iron-lithium complex oxide was recognized for both of S15Cand SPCC until the third day. However, on the fourth day, it changed toa grayish color, which is considered to be a nitrided layer, and thegrayish color in the appearance continued until the eighth day.

[0043] In contrast thereto, the test pieces of S15C and SPCC processedby the salt bath of S2-2 presented a black-colored appearance for all ofthe test specimen from the first day until the eighth day.

[0044] Table 2 shows the results of the salt water spray tests conductedfor the test pieces processed by salt baths of S2-1 and S2-2 inaccordance with JIS Z2371, respectively. TABLE 2 Test Results OfCorrosion Resistance (Salt Water Spray Test in accordance with JISZ2371: Hours until appearance of rust) Salt Day Of Salt Bath TreatmentBath Material 1st 2nd 3rd 4th 5th 6th 7th 8th S2-1 S15C >200 >200 >20048 24 24 24 24 SPCC >200 >200 >200 72 24 24 24 24 S2-2S15C >200 >200 >200 >200 >200 >200 >200 >200SPCC >200 >200 >200 >200 >200 >200 >200 >200

[0045] It was noted that there is a close relation between the corrosionresistance and the external appearance. All of the test pieces havingblack-colored appearance showed satisfactory corrosion resistance.

[0046]FIG. 2 shows a result of analysis measured on the depth from thesurface for the SPCC material treated in the salt bath of eighth day ofS2-2 at 580° C. for 120 min. by means of glow discharge spectroscopy(GDS). As shown in FIG. 2, an iron-lithium complex oxide film of 2 to 3μm thick exists on the outermost layer, and the nitrided layer of about10 μm thick exists under that film.

[0047] In order to investigate a industrial life of the salt bath,theinventors of the present invention proceeded a long term running testwhere the salt bath of S2-2 is further continuously used for a longperiod of time. Like the tests described above, the long term runningtests were carried out by using the same a amount of iron powder and byapplying the same test pieces of iron and steel parts, while thecomposition of the salt bath has been adjusted by supplementing theconsumed component into the salt bath. The processing was conducted fivedays a week, and no processing was made on the weekend. During theweekend, temperature were kept and aeration were maintained.

[0048] In the long term running test of two months, the amount of the byproducted cyanide in the salt bath was approximately at 0.5 wt %, andthe external appearance of the treated metal pieces was black-colored.The results of the salt water spray tests indicated that the time untilappearance of rust is more than 200 hours.

[0049] However, after three months from the start of the long termrunning test, the center and lower portions of the test pieces becamegrayish color, and the salt water spray tests indicated that the timeuntil appearance of rust is shortened to 24 hours or less. The result ofchecking the content of the cyanide in the salt bath showed that thecontent is still maintained at around 5 wt %. However, analysis by X-raydiffraction showed that no iron-lithium complex oxide film was detectedon the surface of the test pieces.

[0050] The inventors therefore started investigation why theiron-lithium complex oxide film that was formed in the early days wasnot appeared after the long term running tests by using the salt bath ofS2-2, in spite of being constantly maintained the contents of thecomponents of the salt bath and the contents of by-producted cyanide.And a part of the molten salt used for the long term running tests wasplaced as samples into a crucible made of titanium having a diameter of110 mm and a depth of 150 mm. And a method to recover the activity toform the iron-lithium complex oxide film was further investigated.

EXAMPLE 2

[0051] The inventors had considered the cause of no formation of theiron-lithium complex oxide film from various points view, whether it isbecause of accumulation of impurities in the salt bath, or whether it isbecause of other reason. As one of the trials, a part of the used moltensalt was taken out and supplemented it with new salt. And aninvestigation was made to find out the suitable ratio to be substitutedby the new salt in order to produce the iron-lithium complex oxideagain.

[0052] As a result, it was found that, when only 15 wt % of the moltensalt was substituted by new salt, then the ability to form theiron-lithium complex oxide revives again. Namely, 15 wt % of the moltensalt used for the long term running tests was replaced with new salt.Then, the carbon steel of S15C and of the cold rolled steel sheet SPCCwere immersed in the salt bath at 580° C. for 90 min. And it was foundthat the test pieces thus obtained showed a black-colored appearance andsatisfactory adhesion, which are distinctive of iron-lithium complexoxide. From this result, it was considered that the ability to form theiron-lithium oxide film has been revived. In the salt water spray testsin accordance with JIS Z2371, it was found that the time untilappearance of rust was longer than 200 hours for these test pieces.

[0053] It was supposed that, if the reason of no iron-lithium oxidecomes from the accumulation of impurities in the salt bath, the amountof substitution of the salt must be at a greater ratio than 15 wt % inorder to revive the ability to form the iron-lithium oxide film.

[0054] Then, the inventors speculated that the reason for the revival ofthe ability to form the iron-lithium complex oxide may be related withanother properties of the newly added salt and not with the old usedmolten salt. Based on this speculation, they have made more wideinvestigation to know the real factor for the revival. The inventorshave paid an attention on the moisture contained in the salt for thesupplement use.

[0055] Inventors provided a dried salt for the supplement use, which wasprovided by being placed the salt in a oven maintained at 300° C. for 5hours (drying loss in this procedure was 3 wt %) in order to evaporatethe free water in the salt. By using this dried salt, 15 wt % of themolten salt used for the long term running tests was substituted. Thesalt bath was kept at 580° C., and iron pieces of S15C and SPCC wereimmersed therein for 90 min. However, in this case, the iron-lithiumoxide film was not formed, and the iron pieces showed grayish appearancethat is considered to be the nitrided layer. Thus, in this case theability to form the iron-lithium complex oxide was not recovered.

[0056] From this result, the inventors thought that the moisture in thesalt bath acted to shift the basicity, namely pO²⁻, of the salt bath tothe basic side, thereby enhanced the oxidizing power of the salt bath,and the ability of the salt bath to form the iron-lithium complex oxidewas revived.

[0057] Incidentally, hydroxide compound such as NaOH, KOH, and LiOH canbe expressed by Na₂O·H₂O, K₂O·H₂O and Li₂O·H₂O, respectively. In orderto confirm the above, the NaOH was added at a rate of 0.3 wt % to thesalt bath used for the long term running tests, then S15C and SPCCsamples were immersed in the salt bath at 580° C. for 90 min. As aresult, it was confirmed that the ability to form the black-colorediron-lithium oxide film was drastically improved.

[0058] Then, a mixture of NaOH, KOH and LiOH prepared by combining eachof them at the mol % indicated in Table 1 was added at a rate of 0.3 wt% to the salt bath used for the long term running tests, and S15 andSPCC samples were immersed in the resultant salt bath at 580 ° C. for 90min. As a result, the ability of forming the black-colored oxide filmwas also drastically revived as in the case where NaOH alone was addedto the salt bath.

[0059] Test pieces to which the black-colored oxide film was formed weretested by the salt water spray test in accordance with JIS Z2371. As aresult, time required until appearance of rust on the surface was foundto be longer than 200 hours for all test pieces.

[0060] From these results of above, the inventors found out the secondreason of not forming of iron-lithium complex oxide film. As explainedbefore, after three months from the start of the long term running test,the center and lower portions of the test pieces became grayish color.However, it was a dry season when three months from the start of thelong term running test in Kanto area where the inventor's laboratoryresides. In the process, air bubbling has been applied to the salt bath.The air used for the air bubbling was natural air without applyinghumidity control thereto. It was understood that, because of lowmoisture content in the air used, the amount of moisture fed to the saltbath was low, which accordingly led to decrease the oxidizing ability ofthe salt bath, thereby causing no formation of the iron-lithium complexoxide film.

[0061] Based on this finding, examination was made in order to find outthe absolute moisture content in the air to be preferably used for thebubbling of the salt bath. As a result, it was understood that the useof air with an absolute moisture content of more than (1×10⁻²kg·H₂O)/(1kg dry air), and preferably more than (2×10⁻²kg·H₂O)/(1 kg dry air), iseffective in order to proceed the nitriding and form the iron-lithiumcomplex oxide film onto the surface of the iron parts.

[0062] The moisture supply to the salt bath is effective to enhance theoxidizing activity of the salt bath used in the present invention.Therefore, moisture supply by water and by steam may result in the goodeffect. However, it is not preferable because the supply of water orsteam into the molten salts being at a high temperature is dangerous.

[0063] As described before, it is advantageous for the formation of theiron-lithium complex oxide film that the amount of the by-productedcyanide in the salt bath is as low as possible. In addition, forminimizing the unfavorable influence against the environment, the amountof the cyanide product in the salt bath should be kept as low aspossible.

[0064] As mentioned in the foregoing, the addition of NaOH, KOH, andLiOH into the salt bath drastically enhance the oxidizing activity ofthe salt bath (it is presumed that the oxidizing activity of the cyanatein the salt bath is enhanced due to increase of the basicity in the saltbath). And even when the accumulated amount of the CN⁻ in the salt bathexceeded 2 wt % level, it is possible to simultaneously form theiron-lithium complex oxide film onto the surface of iron partssimultaneously with the nitriding.

[0065] However, the use of excess amount of alkali hydroxide should belimited to an appropriate extent since it may accelerate thedecomposition of a cyanate, the main component for nitriding. (Whenbasicity of the salt bath became high, the decomposition of the cyanateis accelerated.) The accumulated amount of CN⁻ in the salt bath ispreferably maintained in a range not more than 2 wt %, preferably notmore than 1 wt %.

EXAMPLE 3

[0066] In the example 2, explanation was made on the cause of loss ofthe ability to form the iron-lithium complex oxide film in the salt bathbeing used for the long term and the means to recover the ability.

[0067] The salt bath of the invention is required to be stable forproducing iron and steel parts of good and equal quality in order tomake the invention as a commercial process.

[0068] In this respect, the inventors have investigated on the suitableamount of supplemental alkali hydroxide that has a strong influence onthe oxide film forming ability of the salt bath under the condition ofusing moistened air for the bubbling of the salt bath.

[0069] As described in example 2, the amount of the alkali hydroxideadded to the salt bath for recovering the ability to form theiron-lithium oxide film was 0.3 wt % when the adding salt was NaOH aloneor mixture of NaOH, KOH and LiOH at the mixing ratio indicated in Table1.

[0070] However, further experiments were continued on the amount ofalkali hydroxide to be added. And it was found that an addition of thealkali hydroxide in an amount of 0.005˜0.05 wt % to a total weight ofthe salt bath for each treatment charge enables the salt bath to makethe products of good and equal quality.

[0071] In order to form the iron-lithium complex oxide filmsimultaneously with the nitrided layer, it is required to maintain thecontent of CN⁻ in the salt bath at not more than 2 wt %, preferably notmore than 1 wt %. To comply with this requirement, it is effective tomaintain the content of its parent component, namely CNO⁻, at low.

[0072] Inventors have investigated the nitriding performance of the saltbath of the composition of S2-2 in Table 1 in relation with its contentof CNO⁻, and it was confirmed that the nitrided layer with a normalthickness can be obtained when the salt bath contains at least 5 wt % ofCNO⁻. However, when continuous processing are carried on, the content ispreferably not less than 10 wt %.

[0073] In the conventional salt bath for nitriding process, theoperation are carried out with the CNO⁻ content at around 35 wt %. Inthat case, equilibrated CN⁻ content is in a range of 1˜2 wt % in manycases, though it cannot be fixed to that range since the loss of thesalt may vary depending on the shape and size of the material to beprocessed. Based on the above, it is required to suppress the upperlimit of CNO⁻ content at not more than 35 wt %. And in order tomaintaining the CN⁻ content at 1 wt % or less, it is preferable to keepthe CNO⁻ content to be not more than 25 wt % or less.

EXAMPLE 4

[0074] In the nitriding process, it is important that the salt bath hasa composition to form more preferable nitrided layer.

[0075] In recent years, a nitriding process which arises less thermalstress in the treated metal is required. Therefore, the salt bath ispreferably the one by which the processing at 450° C. can be realized.On the other hand, a cyanate has a melting point lower than that of itscorresponding carbonate. And the inventors prepared a mixed salt for asalt bath for nitriding process containing lithium, sodium and potassiumand having solidifying points of the mixed carbonate of Li, Na and Kbeing to be lower than 500° C., and containing CNO⁻ to be at 10 wt %,and the solidifying points of these samples were measured. The resultsare shown in Table 3. TABLE 3 Solidifying temperature of salt containing10% of cyanate Salt Bath for Nitriding Component S1 S2 S3 S4 S5 C1 C2Li⁺ mol % 25.5 31.0 20.0 45.0 40.0 30.0 30.0 Na⁺ mol % 45.0 26.5 20.025.0 45.0 10.0 55.0 K⁺ mol % 30.0 42.5 60.0 30.0 15.0 60.0 5.0 CNO⁻ wt %10 10 10 10 10 10 10 Solidifying 420 378 388 406 427 483 476 point ° C.

[0076] The carbon steel of S15C and the cold rolled steel sheet of SPCCwere immersed in salt bath at 580° C. for 90 min. The compositions ofsalt bath are shown in Table 3, respectively. Cross sections of theobtained nitrided material were observed with an optical microscope tocheck the thickness of the compound layers and a thickness of the porouslayers formed in the compound layer. The results are shown in Table 4.TABLE 4 Salt Bath for Nitriding and Obtained Compound Layer Salt Bathfor Nitriding Material S1 S2 S3 S4 S5 C1 C2 SPCC CL 10μ CL 11μ CL 8μ CL10μ CL 11μ CL 4μ CL 15μ PZ 0μ PZ 0μ PZ 0μ PZ 0μ PZ 1μ PZ 0μ PZ 8μ S15CCL 12μ CL 12μ CL 10μ CL 13μ CL 12μ CL 6μ CL 19μ PZ 0μ PZ 0μ PZ 0μ PZ 0μPZ 1μ PZ 0μ PZ 8μ

[0077] From the results shown in Tables 3 and 4, it was found out thatthe salt baths of S1, S2, S3, S4 and S5 are recommendable, because eachof these salt baths has a solidifying point of lower than 450° C., andthe nitriding performance, namely the thickness of the compound layer,is more than a normal level and having less porous layer. In contrast toS1 through S5, salt baths of C1 and C2 are not recommendable sincesolidifying points are higher than 450° C., and the salt bath of C1 isinferior in the thickness of the compound layer, and the property of thenitride layer formed in the salt bath of C2 was inferior since itcontained a thick porous layer.

[0078] From the results described above, it was found out preferable touse a salt bath containing alkali components at an containing ratiowhere solidifying temperature of the mixed carbonate of Li⁺, Na⁺ and K⁺falls within a range surrounded by the solidifying temperature line of500° C. in the phase diagram of carbonates of three elements of Li⁺, Na⁺and K⁺ as shown in FIG. 3, and wherein the mol ratio of Na⁺ and K⁺ fallswithin a range from 2:8 to 8:2.

EXAMPLE 5

[0079] <Test for Abrasion Resistance>

[0080] In the embodiment example, specimens of SPCC being treated in thesalt bath of this invention on 8th day of its long term running test ofexample 1 were provided. And the treatment was processed with the saltbath at 580° C. for 90 min..

[0081] In the comparative example, specimens of SPCC material beingtreated by the conventional nitriding salt bath (TAFTRIDE TF1) wereprovided. And the treatment was processed with the salt bath at 580° C.for 90 min..

[0082] Abrasion resistance has been evaluated by measuring the maximumload with no scoring defects by using the SRV testing machine and in thecondition as explained below.

[0083] Holding time: 60 sec

[0084] Step Load: 50N/50 sec

[0085] Slide distance: 2 mm

[0086] Slide frequency: 50 Hz

[0087] Lubricating oil: Base oil for engine oil TABLE 5 Maximum Loadwith no Treatment Process Scoring Defects Embodiment example 1000, 950,1000 Comparative example 750, 850, 900

[0088] From the results shown in Table 5, it is obvious that thematerials processed by the salt bath according to the present inventionare provided with abrasion resistance at least equal to or superior thanthat provided by the conventional nitriding process,

INDUSTRIAL APPLICABILITY

[0089] According to the process of the present invention, iron partshaving excellent corrosion resistance and abrasion resistance can beobtained by carrying out the single nitriding process without requiringan additional electrolysis process.

1. A nitriding process of iron and steel parts having an improvedcorrosion resistance by immersing the iron and steel parts in a saltbath containing cationic component of Li⁺, Na⁺ and K⁺ and anioniccomponents of CNO⁻ and CO₃ ²⁻, wherein an iron-lithium complex oxidelayer are formed at the outermost surface of the iron and steel partssimultaneously with forming nitride layer by using replenishing saltcontaining at least one hydroxide being selected from lithium hydroxide,sodium hydroxide and potassium hydroxide.
 2. A nitriding process of ironand steel parts having an improved corrosion resistance by immersing theiron and steel parts in a salt bath containing cationic component ofLi⁺, Na⁺ and K⁺ and anionic components of CNO⁻ and CO₃ ²⁻, wherein aniron-lithium complex oxide layer are formed at the outermost surface ofthe iron and steel parts simultaneously with forming nitride layer byusing materials of being in a hydrated state or in a free watercontaining state for preparation of new salt or for preparation ofreplenishing salt.
 3. A nitriding process of iron and steel parts havingan improved corrosion resistance by immersing the iron and steel partsin a salt bath containing cationic compounds of Li⁺, Na⁺ and K⁺ andanionic compounds of CNO⁻ and CO₃ ²⁻, wherein an iron-lithium complexoxide layer are formed at the outermost surface of the iron and steelparts simultaneously with forming nitride layer by using air having anabsolute moisture content of more than (1×10⁻² kg·H₂O)/(1 kg dry air)for air bubbling for mixing the salt bath.
 4. A nitriding process ofiron and steel parts according to any of the claims 1˜3, wherein thesalt bath contains 3 cationic components of Li⁺, Na⁺ and K⁺ in ancontaining ratio of being fall the solidifying temperature of the mixedcarbonate of Li⁺, Na⁺ and K⁺ within a range surrounded by solidifyingtemperature contour lines of 500° C. in the phase diagram of carbonatesof these 3 components, and wherein the mol ratio of Na⁺ and K⁺ fallswithin a range from 2:8 to 8:2 and the content of anionic compounds CNO⁻is in a range of 5˜35 wt %.
 5. A nitriding process of iron and steelparts according to any of the claims 1˜3 wherein the accumulated contentof the by-producted cyanid in the salt bath is maintained at less than 2wt % in CN⁻.
 6. A nitriding process of iron and steel parts according toany of the claims 1˜3 wherein the solidifying temperature of the saltbath is in a range of 450˜650° C.
 7. Abration resistant iron and steelparts having an improved corrosion resistance being produced by any theprocesses of claims 1˜3 wherein an iron-lithium complex oxide layer isformed at the outermost surface and a nitrided layer is formedimmediately under the iron-lithium complex oxide layer.